Surface plasmon resonance (SPR) is sensitive to changes in the index of refraction at and near the surface of a metal film (e.g., see A. Szabo, et al., Curr. Opin. Strnct. Biol. 5(1995)699-705). SPR enables an in situ observation of processes occurring between a surface and complex biological solutions that allows, e.g., the acquisition of data in real time without requiring tagging of the analytes. That is a technique convenient for obtaining both kinetic and thermodynamic parameters. Where biomolecules immobilized at a surface lead to changes in the index of refraction of the layer on which they are immobilized, SPR can also detect conformational changes of said biomolecules.
As a typical biosensor chip having this kind of surface, BIACORE which is commercially available from Amersham Pharmacia Biotech., Inc. can be named, which is provided in form of chips in which a matrix of dextran with carboxylated ends is immobilized on a translucent gold film. Such a dextran matrix exhibits a certain extent of resistance to non-specific adsorption of proteins, but because it has a considerable thickness (ca. 100 nm), in certain occasions it may have adverse influence on the thermodynamic and kinetic parameters where, for example, partitioning of the analyte in the matrix takes place.
On the other hand, E. Ostuni, et al., Colloids and Surface B: Biointerfaces 15(1999)3-30 discloses chips comprising a polymer represented by a general formula:HS(CH2)11(OCH2CH2)nO— linker. ligand (wherein n is an integer of 2-7)as carried in the form of a self-assembled monolayer on a gold layer (12 nm) which is applied on a titanium thin layer (1-5 nm) vacuum-evaporated on a glass support. In said review, the authors suggest that the monolayer formed of said polymer having 2-7 ethylene glycol units exhibits resistance to adsorption of proteins. They further suggest that the above result would disagree with the theoretical prediction: for example, when polyethylene glycol (PEG) is grafted on a trichlorovinylsilane-modified glass, the shorter becomes the PEG chain, the more its resistance to adsorption of proteins would drop (S. I. Jeon, et al., J. Colloid Interface Sci. 142(1991)149-158).
However, polymers of small PEG units proposed by E. Ostuni, et al. in certain cases non-specifically adsorb specific proteins (e.g., those having positive electric charge as a whole), under the influence of metallic film surface.